1. Field of the Invention
The present invention relates to a radio communication apparatus such as mobile telephones, portable telephones, and cordless telephones, and more particularly to a radio communication apparatus having an automatic frequency control circuit for controlling a frequency on the basis of a reception frequency.
2. Description of the Related Art
In general, a radio communication apparatus used as a mobile station of a mobile/portable telephone system comprises an automatic frequency control (AFC) circuit. The AFC circuit controls a transmitting channel frequency on the basis of a received channel frequency of a radio signal transmitted from a base station, and is constructed as follows, for example. A radio signal received through an antenna is frequency-converted to an intermediate frequency signal by a receiver, and thereafter demodulated to a baseband signal by a demodulator. A change of the reception channel frequency is detected by a difference detecting circuit on the basis of a signal obtained in the process of receiving and demodulation. The detected value of the change of the reception channel frequency is supplied to a synthesizer. The synthesizer variably controls a transmission local oscillating frequency. As a result, the transmission channel frequency changes following the reception channel frequency. With the radio communication apparatus having this AFC circuit, the transmission channel frequency can be maintained at a stable value on the basis of the accurate radio channel frequency of a signal transmitted from the base station.
Recently, a radio communication apparatus having a digital mode has been developed. In a digital mode, a speech signal is encoded, and a carrier wave signal is digital-modulated by the encoded speech signal and transmitted. In a radio communication apparatus of this type, a modulator/demodulator is constituted by a digital circuit, which includes a difference detecting circuit of an AFC circuit.
The AFC circuit as described above has the following drawbacks:
The synthesizer is operated by an analog control voltage, whereas the difference detecting circuit outputs a digital difference signal. Hence, to operate the synthesizer in response to a digital difference signal output from the difference detecting circuit, a D/A converter for converting the digital differential signal to an analog signal. Since, in general, a D/A converter is large and consumes considerable amount of power, it is difficult to reduce the size of a radio communication apparatus and to increase the continuous operation time.
In addition, if a difference detecting circuit is integrated with a digital modulation/demodulation circuit, the large scale integrated circuit must have a large number of terminals to output a digital difference signal. Hence, it is difficult to make an LSI package compact. For example, to control a frequency with a satisfactory accuracy, 8 to 16 bits are required to transmit a digital difference signal, and providing an integrated circuit with terminals corresponding in number to the bits is considerably disadvantageous to decrease the size of the large scale integrated circuit.